Sheet AA2024‐T3: a new investigation of microstructure and composition

Sheet AA2024‐T3 is probably one of the most studied aluminium alloys in the corrosion field, because, with copper as an alloying addition, it is one of the most corrosion‐prone aluminium alloys. This paper reports new findings on the composition and distribution of intermetallic (IM) particles in AA2024‐T3 through the examination of over 80 000 compositional domains in nearly 18 000 IM particles. This work was achieved by using an electron microprobe to map out 2 × 2 mm² at a step size of 400 nm. This study revealed that the composition of individual particles can vary considerably from ‘accepted’ compositions. Domains within particles were extensive across the surface. Because such a large area was mapped, it was possible to subdivide this area and to look at the variation of particle statistics from region to region, providing some information on the statistical variation for small electrodes. Copyright © 2010 John Wiley & Sons, Ltd.     

SEM and RBS characterization of a cobalt‐based conversion coating process on AA2024‐T3 and AA7075‐T6

Samples of aluminium alloys AA2024‐T3 and AA7075‐T6 were treated with a chromate‐based deoxidizer, then conversion coated with a commercial cobalt‐based solution and finally sealed with a commercial vanadate‐based product. The alloy specimens were examined using scanning electron microscopy, transmission electron microscopy and Rutherford backscattering spectroscopy. The thickness of the cobalt‐based conversion coating increased rapidly up to 5 min of immersion but more slowly for longer times. Sealing resulted in penetration of vanadium through the oxide and a small increase in thickness due to the deposition of a thin sealing coating within the pores and on the external surface of the cobalt‐containing coating. Copyright © 2004 John Wiley & Sons, Ltd.     

Electrochemical behavior of aluminum alloy AA2024 in aqueous solutions in the presence of caffeine

The influence of the presence of caffeine (CFN) on the electrochemical behavior of aluminum alloy AA2024 in aqueous solutions was studied. The interaction between the metal surface and the organic compound is potential dependent as well as time dependent. The anodic currents, responsible for the metal dissolution, decreases in the presence of caffeine even adding chloride anion as contaminant. The EIS data obtained at the open-circuit potential clearly demonstrated that the adsorption of CFN on the surface of the AA 2024 electrode is favored allowing the film defects to be repaired. The protective action of CFN is considerably improved on increasing the adsorption time due to a sealing process which enhances the film stability.     

A combined TEM and SKPFM investigation of the surface layers on rolled AA5050 aluminium alloy using ultra‐microtomy

This article studies the evolution of near‐surface morphology as a function of various thermo‐mechanical treatments along the fabrication line of rolled AA5050 aluminium alloy. Ultra‐microtomy has been used to prepare cross‐sectional thin foils for transmission electron microscopy (TEM) and proper surfaces for scanning Kelvin probe force microscopy (SKPFM) analysis. A slight increase in the Volta potential difference (between the inter‐metallics and the matrix) between the as‐cast surface and the surface obtained after the first hot‐mill pass, emphasized that the changes in surface micro‐structure, which in turn affect the corrosion and electrochemical properties of the finished product, had already occurred at that stage. The Volta potential difference during the subsequent hot‐mill pass remained relatively constant. As far as the near‐surface morphology was concerned, hot‐rolling resulted in the formation of a heavily deformed surface layer. Annealing of the hot‐rolled aluminium sheet resulted in partial re‐crystallization of the surface layer. Subsequent cold‐rolling re‐introduced deformation in the near‐surface region. Copyright © 2008 John Wiley & Sons, Ltd.     

Effect of anodizing conditions on the cell morphology of anodic films on AA2024-T3 alloy

The effects of applied current density, anodizing time, and electrolyte temperature on the cell and pore morphology of anodic films and the voltage-time response obtained during galvanostatic anodizing of AA2024-T3 alloy in sulphuric acid electrolytes have been studied. Scanning electron microscopy was employed to observe the film morphology. Sponge-like porous structure was promoted by anodizing at relatively low current density and high electrolyte temperature. In contrast, linear porous structure was favoured under the converse conditions. Intermediate conditions resulted in films containing either sequential layers of the 2 morphologies or a morphology incorporating features of the 2 types; such conditions were associated with anodizing voltages in the range 25 to 35 V. The reasons for the morphological differences are proposed to be due to interactions between film growth stresses and stresses arising from oxygen evolution on the development of the alumina cells.     

Inhibition effect of cerium in hybrid sol–gel films on aluminium alloy AA2024

Aluminium alloys such as AA2024 are susceptible to severe corrosion attack in aggressive solutions (e.g. chlorides). Conversion coatings, like chromate, or rare earth conversion coatings are usually applied in order to improve corrosion behaviour of aluminium alloys. Methacrylate‐based hybrid films deposited with sol–gel technique might be an alternative to conversion coatings. Barrier properties, paint adhesion and possibly self‐healing ability are important aspects for replacement of chromate‐based pre‐treatments. This work evaluates the behaviour of cerium as corrosion inhibitor in methacrylate silane‐based hybrid films containing SiO₂ nano‐particles on AA2024. Hybrid films were deposited on aluminium alloy AA2024 by means of dip‐coating technique. Two different types of coating were applied: a non‐inhibited film consisting of two layers (non‐inhibited system) and a similar film doped with cerium nitrate in an intermediate layer (inhibited system). The film thickness was 5 µm for the non‐inhibited system and 8 µm for the inhibited system. Film morphology and composition were investigated by means of GDOES (glow discharge optical emission spectroscopy). Moreover, GDOES qualitative composition profiles were recorded in order to investigate Ce content in the hybrid films as a function of immersion time in 0.05 M NaCl solution. The electrochemical behaviour of the hybrid films was studied in the same electrolyte by means of EIS technique (electrochemical impedance spectroscopy). Electrochemical measurements provide evidence that the inhibited system containing cerium displays recovery of electrochemical properties. This behaviour is not observed for the non‐inhibited coating. GDOES measurements provide evidence that the behaviour of inhibited system can be related to migration of Ce species to the substrate/coating interface. Copyright © 2010 John Wiley & Sons, Ltd.     

Effectiveness of 2‐mercaptobenzothiazole, 8‐hydroxyquinoline and benzotriazole as corrosion inhibitors on AA 2024‐T3 assessed by electrochemical methods

In this study, the effectiveness of 2‐mercaptobenzothiazole (2‐MBT), 8‐hydroxyquinoline and benzotriazole as corrosion inhibitors for AA 2024‐T3 aluminium alloy was evaluated. The corrosion behaviour in the presence of each compound was investigated by image‐assisted electrochemical noise analysis, electrochemical impedance spectroscopy, potentiodynamic polarization and the split cell technique. It was found that 2‐MBT has superior inhibition properties compared with the other inhibitors. In particular, the specimens immersed in 3.5% NaCl in the presence of 2‐MBT displayed high values of noise resistance that were maintained for over 400 h of testing, and high values of low‐frequency impedance, measured after immersion for 24 h. The split cell technique and potentiodynamic polarization tests indicated that only 2‐MBT decreases significantly both the anodic and the cathodic reaction rates. Scanning electron microscopy observations and energy dispersive X‐ray measurements complement the findings from electrochemical measurements indicating that only 2‐MBT protects the second phase particles, preventing dealloying, trenching and initiation of corrosion. © 2015 The Authors. Surface and Interface Analysis published by John Wiley & Sons Ltd.     

Oxygen evolution: the mechanism of formation of porous anodic alumina

Abstract  Aluminium anodization behavior in ammonium sebacate solution (w = 4%) in ethylene glycol, and in several H₃PO₄-containing electrolytes, has been investigated. A new mechanism is proposed for the formation of porous anodic films. The model emphasizes the close relationship between pore generation and oxygen evolution. PO₄ ³⁻ ions incorporated in the anodic films behave as the primary source of avalanche electrons. It is the avalanche electronic current through the barrier film that causes oxygen evolution during anodization. When growth of anodic oxide and oxygen evolution occur simultaneously at the aluminium anode, cavities or pores are formed in the resulting films. Accordingly, the mechanisms of growth of barrier and porous films are not very different in nature. These findings are a decisive new step towards full understanding of the nature of anodic alumina films. Graphical abstract        

Trivalent chromium conversion coating on AA2024-T3 used in aeronautical and aerospace industry

Significant research has been conducted to replace the chromium(VI)-based surface treatments, and some commercial substitute systems are now available and needs to be tested to evaluate their performance and to know how they comply with the required specifications. The anticorrosion properties provided by a commercially available trivalent chromium-based product—PreCoat A32—when applied to AA2024-T3 aluminium alloy substrates were evaluated in this work and compared with those obtained with a chromium(VI)-based pretreatment well-recognized reference (Alodine 1200S). The morphology and elemental composition of the conversion coatings were investigated by high-resolution microscopy and energy-dispersive X-ray analysis, respectively, being the wettability of the modified surface measured by contact angle goniometry. The data obtained reveal that PreCoat A32–treated surfaces are more apt to receive aqueous paint schemes than those healed with Alodine 1200S. The corrosion resistance of the treated samples was monitored by potentiodynamic polarisation assays and electrochemical impedance spectroscopy analysis, revealing that PreCoat A32 coatings provide improved corrosion protection for AA2024-T3. The corrosion resistance effectiveness of PreCoat A32 was also confirmed in trials realised in salt-spray chamber, humidity tests, and thermal cycling assays, where more severe exposure conditions were simulated. The gathered data clearly indicate that the PreCoat A32 brings together the mandatory qualities to successfully substitute the conventional and undesirable chromium(VI)-based treatments, in aeronautical and aerospace industry.     

The effect of anode surface area on nanoporous oxide formation during anodizing of low purity aluminum (AA1050 alloy)

Porous anodic alumina layers were obtained by a simple two-step anodization of low purity aluminum (99.5 % Al, AA1050 alloy) in a 0.3 M oxalic acid electrolyte at 45 V and 20 °C. The effect of anode surface area on structural features of nanoporous oxide and process of oxide formation was investigated. An ordered structure composed of nanostripes or nanopores was formed on the Al surface during electrochemical polishing in a mixture of perchloric acid and ethanol. This nanopattern is then replicated during the anodic oxide formation. It was found that the pore diameter, interpore distance, and porosity increase slightly with increasing surface area of the aluminum sample exposed to the anodizing electrolyte. On the other hand, a slight decrease in pore density and cell wall thickness was observed with increasing surface area of the sample. The detailed inspection of current density vs. time curves was also performed. The obtained results revealed that the higher surface area of the anode, the local current density minimum, was reached faster during first step of anodization and the increase in current density corresponding to the pore rearrangement process was observed earlier. Finally, a dense array of Pd nanowires (～90 nm in diameter) was synthesized by simple electrodeposition of metal inside the channels of through-hole nanoporous anodic alumina templates with relatively large surface areas (4 cm²).     

Study of anodizing process on aluminium foam to improve the corrosion behavior

Aluminium foam is obtained by the production of air into metallic melt. This material shows a very low density together with good mechanical properties, high impact energy absorption, and fire resistance. Different production ways to obtain metallic foam are possible. Considering the cost, the Alporas process is particularly interesting. By means of this production method, a block of metallic foam with close cells is obtained. By slicing, foam panels are obtained. The mechanical cut promotes the formation of an open cells texture on the surface. In this last case, the complex morphology of aluminium foam could be a critical point considering the corrosion behavior in aggressive environments, where localized corrosion phenomena, as pitting or crevice corrosion, are likely to occur. The anodizing treatment is one of the most used methods to improve the corrosion resistance of aluminium and aluminium alloys. The aim of this paper is to perform an anodization treatment to enhance the corrosion resistance of aluminium foam. Constant voltage anodization (12 V for 60 min) and pulsed current anodization (0.04 A/cm² for 60 seconds and 0.01 A/cm² for 15 seconds, repeated for 15 cycles) have been carried out in 15 wt% H₂SO₄ at 20°C. The anodized samples are observed in cross section by optical and electronic microscopes to investigate the structure of the anodic oxide layer and the presence of defects and to measure the thickness of the layer. The corrosion protection performance and the compactness of layers are evaluated using acetic salt spray test and electrochemical impedance spectroscopy.     

Active protective Al–Ce alloy coating electrodeposited from ionic liquid

The electrochemical co-deposition of Al–Ce metallic protective coating with active inhibiting effect was performed for the first time using an ionic liquid as an electrolyte. Cerium was successfully co-deposited with aluminium on surface of Pt and AA2024 aluminium alloy forming uniform films with globular micro-structure and thickness up to 75 μm.Cerium was introduced into the aluminium coating as a potential corrosion inhibitor which can be liberated during sacrificial dissolution of the galvanic layer deposited on the alloy surface. The released inhibitor provides an additional active corrosion protection slowing down the corrosion processes in the defects.     

Novel use of glow discharge optical emission spectroscopy (GDOES) to study corrosion of AA2024‐T3 in the presence and absence of inhibitors

Elemental depth profiling by glow discharge optical emission spectroscopy has been used to characterise the corrosion products on AA2024‐T3. In previous work, the aluminium, oxygen and copper depth profiles were shown to provide information regarding surface roughening, the thickness of corroded layers and extent of copper de‐alloying/relocation. In the present work, the study is extended to the detection of corrosion inhibitors deposited on the exposed alloy surface in a model defect. The work includes a comparison of hybrid coatings doped with inhibitors encapsulated in nanocontainers and with the direct addition of inhibitor species to the coating matrix. The work also investigates the effects of inhibitor addition to sol–gel coatings or primer systems or both, highlighting the possible synergistic effects of mixed inhibitor systems. Copyright © 2015 John Wiley & Sons, Ltd.     

Multiscale study of cold-rolling deformation on mechanical and corrosion behaviors of AA2024-T4 aluminum alloy

This study has been conducted to investigate the effects of plastic deformation of an AA2024 aluminium alloy by cold rolling to 25%, 50% and 75% and then heat-treating and naturally ageing for 20 days to T4 on the microstructure and the electrochemical behavior. To characterize the microstructural modifications different techniques have been applied such as X-ray Diffraction (XRD) to demonstrate the intermetallic phases formed, Optical Microscopy (OM) and Scanning Electronic Microscopy (SEM) to evaluate their microstructures and grain size. Moreover, the surface topography has been measured to establish the roughness effect on the mechanical response when subjected to tensile, fatigue and micro-indentation tests. The corrosion behaviour was evaluated by Potentiodynamic Polarization Scanning, Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). The results revealed that cold-rolled samples with 50% plastic deformation show a smoother topography and exhibit the best compromise between mechanical and corrosion resistance.     

Novel use of glow discharge optical emission spectroscopy (GDOES) to study corrosion of AA2024‐T3 in the presence and absence of inhibitors – part 3: investigating the effects of various proportions of inhibitor in the coating matrix or in nanocontainers

Elemental depth profiling by glow discharge optical emission spectroscopy has been used to characterize the corrosion products on AA2024‐T3. In previous work, the aluminium, oxygen and copper depth profiles were shown to provide information regarding surface roughening, the thickness of corroded layers and extent of copper de‐alloying/relocation. Nitrogen, sulfur, phosphorus and chromium depth profiles were examined in the hope of detecting inhibitor species within the corroded/altered layers after 5 h of exposure to a corrosive solution. In the present work, the study is extended to longer exposure time. The work presents a further study of the leaching of benzotriazole from the coating matrix or from nanocontainers during various times of exposure to a corrosive environment. Copyright © 2016 John Wiley & Sons, Ltd.     

Effect of AA2024‐T3 surface pretreatment on the physicochemical properties and the anticorrosion performance of poly(γ‐glycidoxypropyltrimethoxysilane) sol‐gel coating

This study investigated the dependence of the anticorrosion performance of a poly(γ‐glycidoxypropyltrimethoxysilane) (poly(γ‐GPTMS)) sol‐gel coating on AA2024‐T3 aluminum alloy surface state. Two different AA2024‐T3 surface pretreatment procedures were tested: a degreasing with acetone and a chemical multistep etching process (industrial chemical etching pretreatment). Poly(γ‐GPTMS) coatings were deposited onto both pretreated surfaces using the dip‐coating technique. Surfaces were characterized principally by scanning electron microscopy, X‐ray photoelectron spectroscopy, Fourier transform infrared attenuated total reflectance, contact angles, and roughness measurements. Moreover, for the coated AA2024‐T3 surfaces, a pull‐off test was used to evaluate the poly(γ‐GPTMS) adhesion to the pretreated surface. Bare surface properties depended on the applied pretreatment. The chemically etched surface was the roughest and the most concentrated in hydroxyl groups. In addition, comparatively to the degreased surface, it has a more hydrophobic character. Poly(γ‐GPTMS) coating revealed an uneven nature and a poor adhesion once it was deposited onto the degreased surface. Coatings anticorrosion performances were evaluated using electrochemical impedance spectroscopy measurements (EIS). Electrochemical impedance spectroscopy data proved that the sol‐gel coating applied onto the chemically etched surface had better anticorrosion performance.     

Fast formation of thick and transparent titania nanotubular films from sputtered Ti

Ti films sputtered on transparent fluorine-doped tin oxide glass substrates were anodized in fluoride-containing organic electrolyte in the presence of H₂O. In this work, anodic TiO₂ nanotubes (ATNs) as long as 9.2 ± 0.3 μm were obtained with high growth rate of 0.64 ± 0.3 μm min^?1. We demonstrated the optimum anodization conditions for ATN growth on foreign substrates, were within the range of 0.3–0.5% (wt) NH₄F, with 3–5% (vol) H₂O at 60 V. XPS and ICP-MS were utilized to elucidate the increase of thickness and volume expansion obtained from the sputtered Ti film to their ATN forms. The ATN films exhibited excellent uniformity and adhesion to the substrates.     

Novel use of glow discharge optical emission spectroscopy (GDOES) to study corrosion of AA2024‐T3 in the presence and absence of inhibitors

Recent interest in environmentally friendly alternatives to chromate‐based corrosion inhibitors has led to the development of a range of novel coating formulations. The work described herein is aimed at developing a novel methodology to contribute to investigation of the self‐healing and active corrosion protection of the new coatings. An experimental procedure has been developed to model a defect in the coating by fixing coated specimens in close proximity to the uncoated AA2024‐T3, each separated by a narrow gap containing sodium chloride solution. After exposure to the corrosive environment, elemental depth profiles of the uncoated specimens were acquired by glow discharge optical emission spectroscopy (GDOES). The depth profiles of selected elements (notably aluminium, oxygen and copper) were shown to have characteristics which can be correlated with bulk surface roughening/intensity of corrosion, the thickness of the corroded layer and de‐alloying/re‐distribution of copper. An unanticipated inhibitory effect was noted in the case of a coating doped with γ‐Al₂O₃ (γ‐alumina or AluOx). Copyright © 2015 John Wiley & Sons, Ltd.     

The effect of trace additions of copper and pretreatment conditions on the filiform corrosion of powder-coated aluminium alloy AA6060

This paper presents a short review of the literature data and the newest studies of filiform corrosion (FFC) testing of profiles extruded using a recycling-friendly AA 6xxx aluminium alloy, with Cu content up to 0.09 wt%. As the pretreatment plays a significant role in avoiding FFC on powder-coated aluminium profiles, the profiles were pretreated and coated at three different production sites. They have been tested at 40°C, 82% RH for 1000 hours (according to modified ISO 4623-2). It was observed that differences in FFC behaviour of the same type of profile coated at different plants were more significant than differences between samples with increasing Cu content. In order to understand the differences between profiles coated at different sites, GDOES analysis of samples was performed, specially focusing on surface enrichment of copper through the pretreatment.     

Development of cerium‐based conversion coatings on 2024‐T3 Al alloy after rare‐earth desmutting

The deposition of Ce‐based conversion coatings onto 2024‐T3 Al alloy sheet was studied using Rutherford backscattering spectroscopy, scanning electron microscopy, Auger electron spectroscopy, x‐ray photoelectron spectroscopy and atomic force microscopy. The Al sheet was pretreated with an alkaline clean followed by treatment in a Ce(IV) and H₂SO₄‐based desmutter. The Ce(IV)‐based conversion coating solution contained 0.1 M CeCl₃·7H₂O and 3% H₂O₂ and was acidified to pH 1.9 with HCl. Upon immersion, there was an induction period that included activation followed by aluminium oxide growth over the matrix and cerium oxide deposition onto cathodic intermetallic particles and along rolling marks on the surface. After the induction period cerium oxide deposited generally across the whole surface and thickened. The strongest anodic sites initially were adjacent to the intermetallic cathodes and resulted in aluminium dissolution but also oxide thickening. Copyright © 2004 John Wiley & Sons, Ltd.